This is a new R21 application in response to PAR-03-045 "Nanoscience and Nanotechnology in Biology and Medicine." The major goal is to develop a new format of micro- to nano-scale assays for biomolecular interactions such as DNA-DNA, protein-protein, and protein-DNA;we focus exclusively on sequence-specific DNA hybridization for this R21 project. These assays will utilize high-sensitivity Hall crosses, a magnetic sensing technology that has very recently reached the point where we can pursue its use in biomedical applications. An interdisciplinary research team will accomplish this goal through high-risk/high-reward research on three specific aims. (1) Demonstrate selective DNA hybridization by micro-Hall magnetometry with commercially available magnetic beads;feasibility will be established through sequence-specific binding of multiple magnetic particles, each via multiple DNA-DNA linkages. (2) Develop and test nano-scale format Hall magnetometers;sensitivity and signal-to-noise of the assay will be improved upon by parallel development of magnetic nano-particles and further miniaturization of the Hall magnetometer. Nano-particle composition and size will be varied to optimize output of the Hall device. (3) Determine experimentally the limits of bio-molecular detection by Hall magnetometry;detection of sequence-specific DNA hybridization will be tested using the nano-scale Hall crosses and magnetic particles from Aim 2. Nano-particles with optimal composition and size for magnetometry will be tested first for compatibility with bio-molecule functionalization (DNA attachment) and sequence-specific hybridization. Nano-particles that yield optimal combinations of signal-to-noise for Hall magnetometry and sequence-specific DNA hybridization will be tested further in dilute solutions of analyte to determine the limit of detection. Nano-scale magneto-sensing is anticipated for detection of a small number of analyte molecules-one or two molecules in the limit as are needed for single cell analyses. Further, magneto-sensing assays could be used to determine activity of some enzymes such as nucleases and proteases, possibly at the single molecule level. The format should also be amenable to determining the melting temperature for double stranded DNA, again perhaps at the level of a single molecule. Magneto-sensing is also compatible with magneto-control systems, opening up the possibility of controlling (via an externally applied electromagnetic field) the location of bio-molecules attached to beads.